Among optical amplifiers that utilize a semiconductor laser, traveling-wave optical amplifiers are anticipated to be applied in the optical communication industry field in the future, because they have benefits of a wide signal-wavelength band over which they are capable of amplification, and of an extremely fast response-speed with regards to optical signals.
The traveling-wave optical amplifier achieves an amplifying capability by suppressing self-oscillation by the provision of a non-reflective coating on input and output ends of a semiconductor laser; and light injected through a surface of one end of a semiconductor laser chip is amplified inside that chip and appears on the other end as an output. Furthermore, light amplification is performed regardless of the direction of traveling of light inside the semiconductor laser chip, i.e., in any direction in he chip. Therefore, if one of two ends of a semiconductor laser chip is made non-reflective and the other end is made reflective, then light injected from the non-reflective end will be amplified in the semiconductor laser chip, reach the reflective end, and, due to the reflective surface at this end, be reflected to change its direction, whereupon the light will once again be amplified inside the semiconductor chip, reach the non-reflective end, and be emitted outward. An optical amplifier can therefore be obtained, by externally providing an optical circuit having the function of separating light that is input from light that is output.
FIG. 2 is a block diagram showing a prior art example (for example, Japanese Patent Application Kokai Publication No. 224283/1988) of an optical amplifier of this type. 201 is a polarization beam splitter, 202 is a Faraday rotator assembly, 203 is a semiconductor laser having a non-reflective coating 204 at an end and a reflective coating 205 at the other. First, input light having a linearly polarized wave that is P-polarized with respect to the polarization beam splitter 201 is passed through the polarization beam splitter 201 and is optionally rotated 45.degree. by the Faraday rotator assembly 202. The light is then injected into the semiconductor laser 203 and amplified. The amplified light is reflected at the reflective coating 205, amplified again, and emitted outward through the injection end. The light emitted outward is once again optically rotated 45.degree. by the Faraday rotator assembly 202. Therefore, the rotated light has a direction of polarization at an angle of 90.degree. relative to the input light; and light at the output port 207 is S-polarized with respect to the polarization beam splitter 201. In short, the illustrated assembly acts as a reflective-type optical amplifier in which light from the input port 206 is amplified and appears at the output port 207.
However, the optical amplifier described above has a drawback in that there is an amplifying effect only when an input optical signal is incident as P-polarized light with respect to the polarization beam splitter 201: when he light contains other polarization components, that light is reflected by the polarization beam splitter 201 and does not reach the semiconductor laser 203; thereby, the degree of amplification depends on the direction of polarization of the input optical signal; and certain restrictions concerning the direction of polarization of the input optical signal must be met in order for the assembly to achieve the desired function.
For example, when an optical amplifier is used in an optical repeater or the like, the direction of polarization of light propagated in an optical fiber varies due to the physical environment of the optical fiber including pressure on the fiber, bending, or temperature; therefore, the direction of polarization of an input optical signal input to an optical amplifier is not constant. Thus it is unclear with what direction of polarization the light will arrive; and consequently, it is necessary to place a polarization controller at the input stage of the optical amplifier and to operate it in such a way that a P-polarized-wave is always injected. For this reason, there was much hope for an appearance of an optical amplifier having a constant degree of amplification independent of the direction of polarization of an input optical signal.